Override element CSS style

Override element CSS style

Hi all,

My question is, how can i override an element CSS property so it fits my needs?

Explanation :
I have a list item in a tabpanel and and a toolbar that's docked at the top of the page with an absolute position by default. I've changed that position to initial so my list item will be able to scroll behind the toolbar. The reason why i want to do that is because i have an image as a toolbar that contains a transparent background.
This said, after having changed my toolbar position from absolute to initial, my list scrolls well behind my toolbar but there is a space between my list item and my tabbar that i can't get ride of and which the height corresponds to my toolbar height.
I've noticed that when i set the CSS top property of the corresponding panel to zero and set the height at 100% within the "element.style" area of the console panel everything is fine. But i can't override that property in my CSS.